At low temperatures the motion of the atoms is of vibrational nature [1]} and since PEs are related to a local mechanical instability of the particle configuration it can be expected that the latter are associated with quasi-localized soft modes. Various studies support this view, showing that the location of these soft modes are often, but not always, related to the local arrangement of the particles [2]}, [3]}, [4]}, [5]}, [6]}, [7]}, [8]}, [9]}, [10]}, [11]}, [12]}, [13]}, [14]}, [15]}, [16]}.
